It was not a punishment. It was more, "You're sure that you know an efficient way to do this? You get a chance to prove that you're right." It made people careful because being wrong had a consequence, but it usually moved work to the person who thought that they had the best approach.
This happened in Sprint Planning. Obviously by the end of the meeting, everyone would have a full sprint. So volunteering for one thing required giving something else up. This was definitely part of the equation.
People wind up with as much work as they think they can reasonably do. They would have done that whether or not they "just volunteered" themselves. You only wind up with too much if you didn't estimate your own work well.
As for being passive aggressive, the word "just" usually is used in a passive aggressive way. Making people careful about saying it was an improvement. And we all agreed that it was.
Per your description, it is used to stop people from saying or doing something by assigning an uncomfortable unwanted consequence. Someone thinking something is easy does not nor should not imply they actually want it. Most people do not want to do easy tasks all the time.
And no, they did not volunteered themselves. That is just manipulative language.
This happened in Sprint Planning. Obviously by the end of the meeting, everyone would have a full sprint. So volunteering for one thing required giving something else up. This was definitely part of the equation.